1. Field of the Invention
The present invention generally relates to television receivers and, more particularly, is directed to a television receiver for use as a decoder for decoding a high definition television signal in which the width of the baseband has been compressed by a so-called MUSE (multiple sub-Nyquist-sampling encoding) system.
3. Description of the Prior Art
In order to broadcast a high vision signal (i.e., a high definition signal) by one channel of a broadcasting satellite, which has a bandwidth of 27 MHz, the band width of the high vision signal, which has a signal band width of about 20 to 25 MHz, is compressed to about less than 9 MHz according to the FM (frequency modulation) system. To perform such high definition television broadcasting over one channel of the broadcast satellite, the MUSE system was developed which compresses the baseband width of a high definition signal up to 8.1 MHz without deteriorating the quality of the high definition picture.
In converting a video signal from the form of an analog signal to a digital signal in the MUSE system encoder, the necessary sampling points are selected and the other remaining sampling points are discarded (this is a so-called sampling operation), so as to compress the band of the video signal.
The video signal is a three-dimensional signal which may be expressed along horizontal and vertical axes and a time direction axis. The sampling operation can be performed along any one of the three-dimensional axes or along an arbitrary axis which crosses the three-dimensional axes. Further, if a 1/2 sub-sampling is carried out along a certain axis, then the resolution of the video signal in that axial direction is reduced by one half. On the basis of a visual inspection, the resolution appears deteriorated for a moving picture. As a result, in the MUSE system, a motion detecting circuit determines whether each pixel (picture element) is a still pixel or a moving pixel, and a sub-sampling system of the input signal having a sampling frequency of 48.6 MHz is selectively switched.
More specifically, in a still pixel area (i.e., a still picture area), the following succession of operations are performed. That is, field offset sub-sampling (hereinafter, referred to as "VOS") by a clock pulse of 24.3 MHz, interpolation filtering by a frequency of 12 MHz, conversion to a sampling frequency of 32.4 MHz and frame offset sub-sampling (hereinafter, referred to as "FOS") by a clock pulse of 16.2 MHz. If the number of horizontal scanning lines per frame is odd, the FOS becomes equivalent to a line offset sub-sampling (hereinafter, referred to as "LOS") and therefore this FOS can be considered a frame/line offset sub-sampling (FOS/LOS). On the other hand, in a moving pixel area (a moving picture area), the following succession of operations are performed. That is, a band-limiting operation by a low-pass filter whose cut-off frequency is 16 MHz, a conversion to a sampling frequency of 32.4 MHz and processing of offset sub-sampling (LOS) by a clock pulse of 16.2 MHz. In actual practice, a sub-sampling signal of a still image area and a sub-sampling signal of a moving image area at each pixel of an image are generated and summation-mixed in response to the change between frames of the signals of each pixel.
FIG. 1 shows a transmission band of the conventional MUSE system, in which the abscissa axis represents the spatial frequency in the horizontal direction of an image and is expressed in units of a sampling frequency (MHz), and the ordinate axis represents the spatial frequency in the vertical direction of an image, and is expressed in units of c/ph (cycles/picture height) which is the number of horizontal scanning lines per picture. The sampling frequency (MHz) units along the horizontal direction have substantially the same meaning as Msps (samples per second) and, as a result, indicate the number of samples along the horizontal direction per second. As is to be appreciated, a television line is indirectly utilized in the units of spatial frequency in the vertical direction. The relationship between a c/ph and a television line may be expressed as follows: EQU 1[c/ph]=2 ]television lines]
Furthermore, since a high definition signal has 1125 horizontal scanning lines per frame, a sampling frequency of an input signal of a MUSE signal of 48.6 MHz and a field frequency of 60 Hz, on the basis of Nyquist's theorem, the upper limits of the transmission bands of the vertical, horizontal and the time directions are 1125/2c/ph, 24.3 MHz and 90 Hz, respectively.
In FIG. 1, an area 1, having a substantially triangular shape, represents a transmission band of a still image area and a triangular area 2 represents a transmission band of a moving image area. The resolutions in the oblique directions in the still image and moving image areas may be respectively reduced by one half by the offset sub-sampling operation. Further, since one picture is composed of two frames in the still image area and one picture is formed by intra-field interpolation in the moving image area, the maximum value in which an image can be transmitted without frequency distortion (temporal frequency) in a time direction of motion of an image is 1/4 of the frame frequency (7.5 Hz) and 1/2 of the field frequency (30 Hz) in the transmission band 1 of the still image area and the transmission band 2 of the moving image area, respectively. In the band in which the frequency in the horizontal direction in the transmission band 1 of the still image area is less than 4 MHz, aliasing noise does not occur due to the FOS so that the maximum value of the temporal frequency becomes 15 Hz.
In the conventional MUSE system, as described above, since the area in which the temporal frequency of the motion is less than 7.5 Hz is processed as a still image area, the resolution is satisfactory in any of the horizontal, vertical and oblique directions. However, the areas in which the temporal frequency of the motion exceed 7.5 Hz are all processed as moving image areas. In this situation, the transmission band 2 of the moving image area is limited particularly in the oblique direction. More specifically, when oblique lines are formed at a relatively small pitch, approximately 4.sqroot.2 (that is about 6) horizontal scanning lines are vibrated at a frequency higher than 7.5 Hz and, as a result, these oblique lines will be blurred.
To eliminate the above-mentioned disadvantages, a signal having a high resolution with respect to a signal (a middle signal) of an area existing in the band of a predetermined band width may be transmitted with, for example, the motion temporal frequency of 7.5 Hz by improving the MUSE systems band compressing method. However, this proposal is not too practical because it necessitates changing the broadcasting system itself. Accordingly, it is preferable that the resolution of the middle signal be improved by modifying only a decoder (television receiver conforming to the MUSE system) located at the viewer's side without modifying the encoder at the broadcasting station side.